1. Field of Invention
The present invention relates to a scanning-type exposure system for synchronously scanning a mask (such as a photo mask or reticle) and a substrate (such as a wafer or glass plate) relative to a projection optical system, and more particularly to improvement of a focus control method of a projection optical system under scanning exposure and alignment.
2. Description of Related Art
A scanning-type exposure system for synchronously scanning a mask and a substrate relative to a projection optical system is disclosed in, for example, Japanese Laid-Open Patent Application No. 7-57986. A focus control method of the scanning-type exposure system of this type is disclosed in Japanese Laid-Open Patent Application No. 7-273000. Also see U.S. Pat. No. 5,657,130, which claims priority from Japanese Laid-Open Patent Application No. 7-273000.
According to the scanning exposure method disclosed in these references, a pattern on a mask is projected onto a substrate as an erect positive image at one-to-one magnification by a projection optical system. Only a portion of the mask is illuminated at any particular moment, for example, with a slit-shaped illumination area. However, by synchronously scanning the mask and the substrate relative to an illuminating optical system and a projection optical system, the entire pattern on the mask is sequentially projected onto the substrate.
This type of scanning-type exposure system is provided with a focus detection system in order to expose a pattern image on a mask to a substrate at a high resolution. The focus detection system detects the positional information of the mask and the positional information of the substrate at each of a plurality of measurement points that extend in the direction perpendicular to a scanning direction of the mask and the substrate. The average of the measured values is calculated in accordance with the detection results and one or both of the interval (space or distance) between the mask and the substrate and the relative inclination between the mask and the substrate is adjusted. Thus, it is possible to adjust the average planes of the mask and the substrate to the appropriate positional relation conjugate about the projection optical system.
In the case of the above Japanese Laid-Open Patent Applications, focus control is performed during exposure (of the wafer or glass plate) using the technique described above. Therefore, if a substrate is warped, the following problem occurs.
As described above, when focusing during exposure, the average value of detection results of all measurement points of a focus detection system is calculated and the average plane of a mask and of a substrate are adjusted to a predetermined positional relation for a projection optical system in accordance with the average value.
However, the detection point of an alignment sensor that is used for detecting an alignment mark on the substrate is set only at end positions of the substrate, which is different from the measurement points used in the above-described focus detection system. Therefore, when detecting the alignment mark on the substrate by using the alignment sensor, if the plane of the substrate is set by performing the focus control in a manner similar to that used during exposure, the position of the substrate at the detection point of the alignment sensor may be deviated from an optimum focus position.